Solid immersion lens (SIL) technology was developed for fields such as optical microscopy and read/write heads for high-density optical disk drives. A SIL is a lens having a higher magnification and higher numerical aperture than common lenses, where those properties are achieved by filling the space between an objective lens and a target to be observed with solid material having a high refractive index. This concept and technology derives from oil immersion lens technology commonly used with optical microscopes to achieve similar results.
Two types of SIL are commonly known, the hemispherical SIL and the Weierstrass SIL. Absent any other techniques that may be applied in concert with the SIL, the hemispherical SIL theoretically increases the numerical aperture of an optical system by n, the index of refraction of the material of the lens. The Weierstrass SIL is formed of a body of material that is a truncated sphere including more than one hemisphere of material. In the case of a Weierstrass SIL where the height of the truncated sphere is
            (              1        +                  1          n                    )        ⁢    r    ,where r is the radius of the sphere and n is the index of refraction of the material, the numerical aperture can be increased by as much as n2.